Ah that makes sense. I fortunately do not need to use a split on my main amp as it has a separate line level out that is tied to the volume control. And is passes through the same tube preamp stage of the amp, which is giving the plate amps the same sonic signature as the mains (also a good thing). I haven't done any measurements yet, but they seem pretty to integrate.
I did get a pair of the fMod line level passive HP filters, but I don't see any way to make those work with my amp without a splitter. It would be nice to free the amp from having to reproduce those low frequencies, but I don't know what trade off is better (run the tube amp full range and use the line outs to the plate amps, giving the plate amps the same tube sig. as the mains) vs. getting the extra power out of the tube amp. I'll play around with it.